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7.2.2.3 Detailed Design Procedure

A major component in designing the LCD solution is determining the exact connections between the LCD_C peripheral module and the display itself. Two basic design processes can be employed for this step, although in reality often a balanced co-design approach is recommended:

PCB layout-driven design

Software-driven design

In the PCB layout-driven design process, the segment Sx and common COMx signals are connected to respective MSP430 device pins so that the routing of the PCB can be optimized to minimize signal crossings and to keep signals on one side of the PCB only, typically the top layer. For example, using a multiplexed LCD, it is possible to arbitrarily connect the Sx and COMx signals between the LCD and the MSP430 device as long as segment lines are swapped with segment lines and common lines are swapped with common lines. It is also possible to not contiguously connect all segment lines but rather skip LCD_C module segment connections to optimize layout or to allow access to other functions that may be multiplexed on a particular device port pin. Employing a purely layout-driven design approach, however, can result in the LCD_C module control bits that are responsible for turning on and off segments to appear scattered throughout the memory map of the LCD controller (LCDMx registers). This approach potentially places a rather large burden on the software design that may also result in increased energy consumption due to the computational overhead required to work with the LCD.

The other extreme is a purely software-driven approach that starts with the idea that control bits for LCD segments that are frequently turned on and off together should be co-located in memory in the same LCDMx register or in adjacent registers. For example, in case of a 4-mux display that contains several 7-segment digits, from a software perspective it can be very desirable to control all 7 segments of each digit though a single byte-wide access to an LCDMx register. And consecutive segments are mapped to consecutive LCDMx registers. This allows use of simple look-up tables or software loops to output numbers on an LCD, reducing computational overhead and optimizing the energy consumption of an application. Establishing of the most convenient memory layout needs to be performed in conjunction with the specific LCD that is being used to understand its design constraints in terms of which segment and which common signals are connected to, for example, a digit.